Local area networks are widely used for communicating among data processing systems and peripherals. As is well known to those having skill in the art, local area networks can use one or more of many network protocols for passing data along the network. In the computer integrated manufacturing environment, two widely used network protocols are the LAT protocol and the Data Highway/Data Highway Plus (DH/DH+) protocol.
As is well known to those having skill in the art, LAT is a name given to Digital Equipment Corporation's (DEC) Ethernet-based terminal server networking protocol. One or more host computers, typically a DEC or DEC compatible computer, communicates with one or more terminal servers over a DEC Ethernet network using LAT protocol at rates of up to 10 megabits per second. LAT packets are passed between DEC hosts and their terminal servers on the DEC Ethernet network. The data in the LAT packets use the DF1 data format when communicating with an Allen-Bradley DH/DH+ network. The DF1 format conforms closely to the ANSI X.28 standard. It combines the features of subcategories D1 (data transparency) and F1 (2-way simultaneous transmissions with embedded responses).
Another widely used local area network in computer integrated manufacturing systems is the Allen-Bradley Data Highway/Data Highway Plus (DH/DH+) network. The DH/DH+ network allows peer-to-peer communication among up to 64 nodes using a half-duplex polled protocol and rotation of link mastership, and operates at a data rate of 57.6 Kbaud. The DH/DH+ network is used to connect Allen-Bradley Programmable Logic Controllers (PLC) in a network. The DH/DH+ protocol is described in a reference manual entitled Data Highway/Data Highway Plus Protocol and Command Set, publication 1770-6.5.16, November 1988, published by Allen-Bradley Company, Inc., Milwaukee, Wis., the disclosure of which is hereby incorporated herein by reference.
Because of the widespread use of the DEC Ethernet network and the Allen-Bradley DH/DH+ network in computer integrated manufacturing environments, there is a need for a bridge between the networks. Such a bridge would connect the two networks to allow two-way communications between host computers running on the DEC Ethernet network, and Allen-Bradley programmable logic controllers running on the DH/DH+ network. Both the DEC Ethernet network and the Allen-Bradley Data Highway/Data Highway Plus network are high speed networks, with communication speeds of 10 megabits per second and 57.6 Kbaud respectively. Accordingly, in order to avoid a communications bottleneck between the two networks, it is essential that the bridge operate at high speed.
Unfortunately, to the best of the inventors' knowledge the only known bridge between the two networks produces a communications bottleneck therebetween. In particular, DEC presently markets a hardware link between the two networks in the form of a DEC terminal server which is connected to an Allen-Bradley 1785-KE module or an Allen-Bradley 1770-KF2 module. Unfortunately, the Allen-Bradley 1770-KF2 module is only capable of transmitting and receiving data asynchronously at 9600 baud, while the Allen-Bradley 1785-KE module has a 19.2 Kbaud limit. This is much lower than the 10 megabit/see and 57.6 Kbaud rates of the networks themselves. Accordingly, communication between the networks is limited, resulting in equipment capacity limitations and an inability to accurately control the equipment connected to the networks. The DEC terminal and KE/KF2 modules are also relatively expensive, so that it is not cost effective to eliminate the bottleneck by adding multiple DEC terminal servers and/or multiple KE/KF2 modules between the networks. Moreover, multiple KE/KF2 modules create excessive overhead on the network, thereby slowing overall communications.